ABSTRACT
Many stars exist in binary and even in multiple star systems. Only in sys tems that are close to us can we observe the separate components directly. Astrophysicists today believe that the number of binary systems is still greater than what we now observe. New binary systems are discovered to this day, even among bright stars whose existence we have known for long time. When the mass ratio between the companions is large, it is difficult to observe the binarity, although even very low mass companions can in fluence the system significantly. Many phenomena in stellar spectra that could not be satisfactorily explained in the past were later found to derive from binarity.
